The laying of asphalt paving material on road surfaces involves the spreading of paving material that consists of a heated aggregate filled bituminous mixture on a prepared roadbed. The paving material is spread while hot and is then compacted so that upon cooling a hardened pavement surface is formed. Conventional paving machines utilize a heavy assembly, termed a “screed”, which is drawn behind the paving machine. The screed typically includes four replaceable screed plates that are constructed of a suitable steel, to spread a smooth even layer of paving material on the prepared roadbed. The weight of the screed assembly aids to compress the paving material and perform initial compaction of the paving material layer. screed assemblies can include vibratory mechanisms placed directly on the screed plates or separate vibratory tamper bars connected in tandem with the screed plates to aid in the initial compaction of the paving material.
To facilitate a proper deposition of the paving material, the screed plates are typically heated, to a temperature in the range of from about 82° to about 171° C. (180° to 340° F.). Heating the screed plates assists the paving material in flowing under the screed plates and reduces adhesion of the paving material to the screed plates. If the screed plates are not adequately heated, the bituminous mixture contacts the bottom of the screed plate and begins to harden, resulting in buildup of paving material and excessive drag. If the temperature is too high, the screed plates may warp, the heated paving material may be damaged or components associated with the screed plates may be damaged. Generally, screed plates are heated to a temperature close to the temperature of the heated asphalt material.
Some conventional screed assemblies are commonly heated by fossil fuel powered burners that heat the upper surface of the screed plate by the direct application of flame or hot exhaust gases. The use of fossil fuel burners to heat screeds has drawbacks. For example, the combustion of fossil fuels generates smoke that represents a source of environmental pollution, and creates a poor working environment for the paving workers. Further, because the flames or exhaust gases of the burners actually contact the surfaces of the screed plates, warping may result. The contours of the screed plates determine the quality, evenness or smoothness of the paving material that is being laid down. While screed plates are often flexed under extreme tensile loads during use to achieve desired crowning or other surface contours, warping of a screed plate can render it useless.
Elastomeric electrically powered heating pads positioned on the upper surface of the screed plates with layers of insulation placed on top of the heating pads have also been used. A heavy steel grid member is placed on top of the insulation to hold the heating pads and insulation in place. However, the elastomeric material typically has poor resistance to tear, abrasion, and poor to fair resistance to fluids such as oil, gasoline and solvents. Additionally, this design requires loose components placed on top of one another to maintain full contact of the heating pads with the screed plates.
Other more sophisticated electric heating systems for screed plates are available. Typically, these heating systems are electric and include temperature sensors, which are used in a feedback control system to maintain the screed plates at the appropriate temperature or within the appropriate temperature range. One problem associated with this design is the tendency for temperature sensors to fail from time to time. On many systems, the user can bypass the temperature sensor and heat the screed plate in an open-loop configuration until a repair can be made. In short, the user removes or bypasses the temperature sensor to override the system thereby enabling the user to continue to pave. However, removing or by passing a temperature sensor can lead to potential overheating of certain components in the system beyond the allocated design limits. As a result, repairs in addition to replacement of the failed temperature sensor may be needed. Further, the manual override technique may raise certain warranty issues, particularly if components unrelated to the screed plate heating system are damaged by the excessive heat.